Issue with HEATER4 (U10 74HCT02 issue?)

I have a DuetWifi + Duex5. For now, I just use 1 heater, connected to the first heater output of the Duex5.

I wanted to test another hotend, so I configured and connected it on the second heater of the Duex5. But it does not work.

At first, the led was not lighting, so was thinking to a config problem. So I testes all other heaters, and they work fine. I then check the signals: not voltage on the MOSFET. Then, I checked my expansion port bridge: all is fine.

So, I wanted to test if the DuetWifi was correctly sending the signal to the 74HCT02 buffer (U10): as soon as I touched the pin 8 with my multimeter wire (other one was not connected to ground), the led turned on (and now always stays on). There is 0.3V on this pin. And I didn't make any short, I'm sure of that (pin 8 is well located, easy to touch by its side).

I disconnected the expansion connector and measured the HEATER4 signal coming from the DuetWifi: 3.3V.

Do you confirm this is an issue with the buffer? For some reason, it shorts the DuetWifi signal to almost 0V, so turn on the FET.

It could also mean there is a bad connection in the cable, or a bad soldered joint on pin 8 of the 74HCT02, or a bad soldered joint on the SAM4E on the Duet; so that the 74HCT08 pin isn't actually connected to the pin on the MCU. When you touched your multimeter to the 74HCT02 you connected it to ground, whereas previously it was floating.

My money is on a bad connection in the cable. But check the soldering on the 50-way connectors of the Duet and DueX5 too, especially if you have an early Duet because the early ones had that connector hand-soldered so a pin could have been missed.